The fundamental purpose of a fiber optic connector is the non-permanent connection of an optical fiber to another optical fiber, to an active device such as an emitter or detector, or to a passive device such as a branching device or attenuator. A connector terminates the fiber such that interconnection is facilitated. The basic purpose of the connector is to position two optical fiber cores with a very high degree of mechanical precision. Fiber optic interconnection must reliably and repeatably align components with micrometer level or even submicrometer level precision and yet also achieve the economies of mass production.
The objective of a connector system is maximum efficiency in power coupling across the joint, for example, from fiber core to fiber core. Of the parameters describing an optical joint using connectors greatest attention is usually paid to insertion loss. The contributions to joint loss are both intrinsic and extrinsic to the fiber or fibers. Fiber intrinsic losses stem from differences in such parameters as core or mode-field diameters or numerical apertures between the two fibers being joined. Prior connectors have been unable to alleviate most intrinsic losses. Extrinsic losses, on the other hand, result from the effects of the optical quality of the fiber's end, index matching, cleanliness, and various forms of misalignment. The history of fiber optic connector technology revolves around the understanding and minimization of these extrinsic effects.
The control of optical fiber connector losses is based on control of three mechanical conditions of the mated fiber ends: lateral displacement, fiber end separation and angular alignment. The largest contributor to connector loss is the lateral displacement of the fiber cores. The second largest contributor to conductor loss is end separation of the fiber cores. Theoretically the most desirable situation is to have the connector end faces just touching. In practice precision plastic or metal ferrule components do not always allow this condition to be reliably implemented. Least critical in its contribution to optical loss is the angular alignment of the fiber ends.
In addition to providing an acceptably low insertion loss a connector system must also provide good repeatability of this loss over many disconnect/remate cycles. In certain applications repeatability is even more important than nominal loss. Additionally, the loss must not be overly sensitive to temperature, vibration or cable tension.
Fiber optic connectors can be generally grouped into three major classes: "bare-fiber" connectors, ferrule connectors (straight and conical), and expanded-beam connectors. "Bare-fiber" connectors are essentially temporary mechanical splices and have not proven commercially viable. Ferrule connectors, by comparison, protect the fiber by encasing it in a cylinder or cone many times its diameter. If the fiber's core can be positioned accurately on the centerline of the ferrule, the jointing problem is simplified to aligning two larger objects, that is, the ferrules instead of the fibers. The mechanical problems of ferrule connector schemes therefore lie in the concentricity of fiber core to ferrule outer surface and concentricity of one ferrule to the other. Both are related to diameter tolerances. Ferrule connector designs have been overwhelmingly cylindrical. Perhaps the best known of these is the so-called SMA style. However a conical ferrule is in widespread use in telecommunications and elsewhere. With a tapered design no sliding fits are needed and diametral tolerances are alleviated. Since two mating conical ferrules are joined in an interconnecting bushing with a biconical internal shape the system is called "biconic."
Expanded beam connectors columnate the rays of the mating fibers, creating virtual "core" diameters much larger than those of the fibers themselves. This approach greatly diminishes the effects of radial misalignment and end face contamination. On the other hand, they are more sensitive to angular misalignment and, as a result, trade one set of problems for another.
The SMA style connector finds large scale usage in data communications environments. The AT&T ST connector also is in significant use in the data communications field. Another SMA style connector in growing use is the Amphenol ST type product using ultra precise cast ceramic ferrules. For connectors to be used in military and aerospace applications reliability and standardization are the key features. The SMA style single channel connector has been popular and is now described in a MIL standard.
Summaries of the background state of the art are found in "Ceramic Capillary Connectors", "Photonics Spectra" October 1984, pages 65-70 and "Connectors: Trends", Laser Focus/Electro Optics June 1987, pages 130-146.